There are several UMA-enabled phones each from Nokia, Samsung, Motorola, as well as from ODMs like Chi Mei, Benq and others. These are more like low to medium priced feature phones using embedded operating systems like Nucleus or RTEMs than high-end smartphones like the Nokia E61 or Samsung SGH-i600 (not the Blackjack) that run "open" operating systems like Windows Mobile or Symbian S60 r3. The memory footprint and processor speeds of these UMA-enabled phones are optimized for cost, not performance or flexibility. The bottom line is that one should not expect to run Skype or a SIP client on these handsets.

UMA is essentially GSM and GPRS over WiFi. T-Mobile and other service providers are taking advantage of the fact the customer already is paying for Broadband and have a WLAN.

That being said, the benefit to the end customer-- the user-- are excellent: in-building coverage is great and that services they might normally not use, such as downloading ringtones, video or music, work much faster at 11MB/s than at 46KB/s and are now a lot less painful to use.

This is something that I think is awesome. For starters, T-mobile's UMA will give them a much-needed boost as far as competing in the US mobile phone arena.

Also, it will (hopefully) prompt more manufacturers to add UMA capabilities to their wifi-enabled cellphones. Why it's not already standard is beyond me. I suppose because it's not a readily available service.